Re: keyword extern
From: tweak (xbwaichunasx_at_cox.net)
Date: 06/26/04
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Date: Sat, 26 Jun 2004 13:39:06 -0700
Jack Klein wrote:
> On Sat, 26 Jun 2004 10:19:23 -0700, tweak <xbwaichunasx@cox.net> wrote
> in comp.lang.c:
>
>
>>Emmanuel Delahaye wrote:
>>
>>>In 'comp.lang.c', tweak <xbwaichunasx@cox.net> wrote:
>>>
>>>
>>>
>>>>What's the best way to use extern when using multiplefiles that is
>>>>easiest to maintain?
>>>
>>>
>>>First of all, try to avoid the use of global scope variables. Read-only are
>>>acceptable, but read/write are source of trouble. However, if you insist ...
>>>
>>>
>>>
>>>>Is it best to declare:
>>>>
>>>>extern int a;
>>>>
>>>>in a header file and include the header file in all files except
>>>>where it's defined.
>>>
>>>
>>>Almost. The header should also be included in the definition source file.
>>>This is the only way to check the match between the declaration and the
>>>definition.
>>>
>>>
>>>/* data.h */
>>>/* usual guards ommited */
>>>extern int x;
>>>
>>>/* data.c */
>>>#include "data.h"
>>>long x; /* ERR! */
>>>
>>>It also allows nice things like :
>>>
>>>/* data.h */
>>>/* usual guards ommited */
>>>extern int a[128];
>>>
>>>/* data.c */
>>>#include "data.h"
>>>int a[];
>>>
>>>The size definition is now unique, public and centralized. It helps reading
>>>and maintenance.
>>
>>Thanks. I'm just trying to understand best practice use of the keyword
>>extern since I do not program for a living, but I would like to start a
>>project at source forge this year. And I am just double checking the
>>basics.
>>
>>So the best practice would be to use global static variables in practice
>
>
> Note that the word "global" does not exist in the C standard. The
> term you are looking for here is "file scope" which is well defined by
> the C standard.
>
> All declarations and definitions written outside of any function in a
> file have "file scope". Their scope lasts from the point of the
> definition or declaration until the end of the translation unit being
> compiled.
>
> File scope definitions and declarations have external linkage by
> default, unless the keyword static is added.
>
> One never needs to use the extern keyword with a function definition,
> prototype or declaration. All references to functions have external
> linkage by default.
>
> One needs to us the extern keyword for declarations (not definitions)
> of data objects that are defined elsewhere. That is because of the
> tentative definition feature in C.
>
> If, outside of any block in a file, you have the three file scope
> declarations:
>
> extern int x;
> int y = 0;
> int z;
>
> ...then here is how a C compiler understands them.
>
> -- extern int x; If there is no other file scope declaration of this
> object in the source file, the compiler will treat it as a simple
> external declaration. If any code in the source file access 'x', the
> compiler will generate an external reference for it. In the final
> stage of creating the executable program, usually called linking, the
> int 'x' must be provided by another source file or library.
>
> -- int y = 0; This is a definition of an int named 'y' that has
> static storage duration, external linkage, and file scope. It will be
> initialized with the value 0 before main() is called, and its lifetime
> will be the entire execution time of the program. Even if none of the
> code in the source file access 'y', the compiler will generate an
> external definition for it. If there is more than one external
> definition of this object in the final program, the result is
> undefined.
>
> --- int z; This is a 'tentative definition'. If there is no other
> declaration for this object, then at the end of the file the compiler
> will automatically create a definition "int z = 0'" and create the
> object.
>
>
>>rather than modifiable extern variables? How should extern be used then?
>
>
> As to the use of variables with external linkage, modifiable or not, I
> will say this: All absolute rules are rubbish, including this one.
>
> Modifiable objects with wide scope, whether they have external linkage
> or not, can be and often are abused and become the cause of hard to
> find program defects. Nevertheless, like almost every other feature
> of the language, they can be used properly and sometimes they are the
> best solution to a problem. On the other hand, many times there are
> better solutions possible.
Thanks for being very clear.
I am trying to avoid problems of maintenance before I finish writing my
project. I see too many open source projects, where I cannot easily
navigate through the code, and I'm trying to avoid that from the onset.
I saw extern as being a probable maintenance problem since tracking down
definitions has never been any fun.
Thanks Again,
Brian
P.S. I'm still learning.
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